Timely and regulated repair of broken DNA is a crucial barrier to cancer formation. My sponsor laboratory has recently discovered and characterized the Shieldin (SHLD) protein complex, a novel regulator of repair which plays a crucial role in determining whether DNA double-strand breaks (DSB) are repaired by homologous recombination (HR) or non-homologous end joining (NHEJ), the two major cellular pathways of DSB repair. SHLD acts by directly inhibiting HR, thereby pushing the balance of DSB repair toward the mutagenic NHEJ pathway in specific contexts where it has been recruited by the chromatin binding proteins 53BP1 and RIF1. However, my preliminary data suggests that SHLD may be ?hijacked? by inappropriate expression of the normally meiosis-specific gene HORMAD1, leading to an unrestricted and mutagenic inhibition of recombination outside of normal SHLD-recruiting chromatin domains. HORMAD1 expression is aberrantly activated in up to 5% of breast cancers (METABRIC) and enforces a BRCA1-mutant-like state of recombination deficiency and sensitivity to pharmacological PARP inhibitors (PARPi); HORMAD1 expression may therefore also be a useful biomarker for cancers that are sensitive to the clinically approved PARPi olaparib. Interestingly, one of the previously described roles of HORMAD1 in meiosis is to inhibit undesirable recombination events between sister chromatids. It is therefore likely that my studies of a HORMAD1-SHLD complex in cancer will also provide insights into essential regulatory mechanisms at work during meiotic recombination. The specific aims of this project are thus to: 1) characterize the composition and regulation of HORMAD1-containing SHLD (mSHLD) complexes in mitotically dividing cancer cells and, 2) study the contribution of mSHLD to the regulation of meiotic recombination and gametogenesis in newly-generated mouse models. This strategy will necessitate the acquisition of technical and theoretical skills that will be of significant benefit in supporting my independent career, including experience with a new model system (mice) and a new field of study (meiotic recombination). Importantly, my sponsor laboratories balance these requirements with the training, expertise, mentorship, collaborators and institutional support that ensures extensive career development during the training period. In the short-term, this will result in significant insights into a novel source of genome instability that may be both a clinically useful biomarker and an important contributor to human fertility. In the long-term, these findings and the skills acquired in making them may also provide the foundation of my independent career.